=IF(OR(A1="",B1=""),"",formula)

will display as empty if either start or end are blank


--

Regards,

Peo Sjoblom


"Pete" wrote in message
...
Thanks again.

BTW I find that the NETWORKDAYS is inclusive so has to be adjusted by
'-1'.

One final question: is it possible to populate the relevent columns with
the
formulas without them showing up until until values are entered into the
adjacent cells?